In recent years, with the increasing scarcity of petroleum resources, the crude oil has become more and more heavy and inferior, and the contents of S, N, O, and metals in the middle distillate obtained through distilling crude oil have increased accordingly. However, the provisions on the contents of S, N, O, and metals in fuel oils specified in the laws and regulations in different countries of the world become harsh increasingly. Hydroprocessing is a common means for removing S, N, O, and metal impurities in hydrocarbon and improving the quality of middle distillate.
In the conventional hydroprocessing using trickle-bed reactor, a large quantity of circulating hydrogen gas and the corresponding circulating system are required to take away the reaction heat and inhibit carbon deposition and coke formation on the catalyst. Consequently, on one hand, the apparatus for hydrogenation reaction is bulky; on the other hand, the capital cost and energy consumption of the apparatus for hydrogenation reaction are increased.
In the hydroprocessing using a trickle-bed reactor, it is needed to transfer hydrogen gas from a vapor phase into a liquid phase, where the hydrogen gas is absorbed along with the reactants to the surface of a catalyst, and thereby the reaction happens under the action of catalyst activity center. Such a mass transfer process of hydrogen gas undoubtedly has adverse effects to the reaction rate of the hydroprocessing.
In view of the drawbacks in the conventional hydroprocessing using trickle-bed reactor, the researchers have developed a liquid phase hydrogenation process.
U.S. Pat. No. 6,428,686 discloses a hydroprocessing method, comprising mixing fresh raw oil with a diluent and a large quantity of hydrogen gas, separating excessive gas from the obtained mixture in a gas-liquid separation unit, and then feeding the mixture into a reactor so as to contact with a catalyst and perform hydrogenation reaction. The diluent is a substance in which the solubility of hydrogen gas is high (for example, circulating hydrocracked product or isomerized product), whereby to increase the amount of hydrogen gas carried in the hydrocarbon feed, as a result, the demand for the circulating hydrogen gas is eliminated.
The major procedures of the liquid-solid phase hydrogenation method for hydrocarbon oil disclosed in CN101280217A and CN101787305A comprise mixing fresh raw oil, circulating product, and hydrogen gas in an supersaturated amount, treating the obtained mixture by gas liquid separation in a gas liquid separation unit, and then feeding the mixture into a hydrogenation reactor to contact with a catalyst and perform reaction.
Although the demand for circulating hydrogen gas is eliminated in the liquid phase hydrogenation methods described above, all of these methods still have the following drawback: a diluent or circulating product is required to increase the amount of hydrogen gas carried in the feed, as a result, the throughput capacity of the hydrogenation apparatus for the fresh raw oil is decreased, which has an adverse effect on the production efficiency.
Therefore, there is an urgent demand for a method for hydroprocessing hydrocarbon oil in liquid phase without the aid of any diluent or circulate product, so as to simplify the process route, reduce capital cost and operation cost, and realize efficient and energy-saving industrial production.